Allow Webrat to read the data-method attribute

Rails 3 will contain unobtrusive JavaScript helpers, allowing
easy switching between JavaScript frameworks. By making use of the
new data attributes of HTML5, Rails stores information about the
action to take. In case of a link with a delete HTTP method, Webrat
won't work:

link_to "Delete", instance, :method => :delete

In Rails 3, this will produce the following HTML:

<a href="/instance/123" data-method="delete">Delete</a>

The following monkey patch makes use of the "data-method"
attribute and allows Webrat to follow delete links in Rails 3.